Star of the Guardians Series by Margaret Weis

As a corrupt Commonwealth rules the galaxy through the might of its armies, its most influential general--a renegade Guardian of the deposed Starfire royal line--pursues the rumor of a hidden heir to the throne and searches for a woman he loves and is destined to destroy.From the Trade Paperback edition.

By calling a temporary truce, Derek Sagan and the rebels thwarted the alien Corasian invasion. Enemies once again, the rebels have resumed their defiance and Sagan has retumed to his campaign to topple the corrupt galactic government.  He plans to set up Dion as king of the Starfire dynasty--and to place himself as the ruling power behind the throne.  On a remote planetary sinkhole of sin and corruption, a small weapon-barely ten centimeters on a side--is hidden... moreBy calling a temporary truce, Derek Sagan and the rebels thwarted the alien Corasian invasion. Enemies once again, the rebels have resumed their defiance and Sagan has retumed to his campaign to topple the corrupt galactic government.  He plans to set up Dion as king of the Starfire dynasty--and to place himself as the ruling power behind the throne.  On a remote planetary sinkhole of sin and corruption, a small weapon-barely ten centimeters on a side--is hidden.  If activated, this seemingly harmless crystal cube could tear a hole in the universe. . .and destroy the fabric of creation.  Sagan wants it.  Lady Maigrey wants it.  And so does Abdiel, a cruel genius who commands a drugged army of mindless slaves. And now Dion is caught in this momentous struggle as he faces his greatest trial yet in his battle to gain the interstellar throne.From the Paperback edition. less

The spellbinding conclusion to the star-spanning saga of adventure and intrigue. At the end of King's Test, Sagan--the man who overthrew Dion's father--pledged his allegiance to Dion as the new king. Now, Dion battles alien and human enemies, and must give up both his lover and one of his men as he learns what it truly means to be king.

Young Dion Starfire is the ruler of a galaxy that is finally at peace after years of strife and bloodshed.  Yet the peace is an uneasy one.  Dion has fallen desperately in love with a woman who is not his queen, and suddenly the fragile alliances that rest of his marriage are threatened.  Then real violence erupts as an illegitimate son of the dead king, in hiding on a forgotten planet, plots Dion's overthrow... moreYoung Dion Starfire is the ruler of a galaxy that is finally at peace after years of strife and bloodshed.  Yet the peace is an uneasy one.  Dion has fallen desperately in love with a woman who is not his queen, and suddenly the fragile alliances that rest of his marriage are threatened.  Then real violence erupts as an illegitimate son of the dead king, in hiding on a forgotten planet, plots Dion's overthrow.  At his command is an army of unseen "ghosts"--alien presences that can roam the galaxy and kill at will.  Dion must turn to an old mentor and enemy, Derek Sagan, as the one man who can help him battle the bastard prince and his dark minions.  And at Derek's side hovers the powerful, shadowy presence of his lost love, Lady Maigrey.  If Dion can only win their aid, he may have one last chance to preserve his throne--and peace for the galaxy.From the Paperback edition. less